Prior to the present invention, various one and two-package moisture curable room temperature vulcanizable (RTV) compositions were available based on the use of a silanol-terminated polydiorganosiloxane having the formula, ##STR1## where R is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, which is preferably methyl, or a mixture of a major amount of methyl and a minor amount of phenyl, cyanoethyl, trifluoropropyl, vinyl, and mixtures thereof and n is an integer having a value of from about 50 to about 2500, with a cross-linking silane having hydrolyzable radicals attached to silicon.
Ceyzeriat, U.S. Pat. No. 3,133,891 and Bruner, U.S. Pat. No. 3,035,016, are based on the use of methyltriacetoxysilane with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions. Although the one-package compositions of Bruner or Ceyzeriat, upon exposure to atmospheric moisture, provide satisfactory one-package room temperature vulcanizable organopolysiloxane compositions exhibiting satisfactory tack-free time, for example, 30 minutes or less after an extended shelf period, the acetic acid by-product is corrosive and has a disagreeable odor.
Other variations of one-package acyloxy acid generating RTV's are shown by Kulpa, U.S. Pat. No. 3,296,161, Goossens, U.S. Pat. No. 3,296,195 and Beers, U.S. Pat. No. 3,438,930, assigned to the same assignee as the present invention. Additional one-package acyloxy acid generating RTV compositions are shown by Schulz et al, U.S. Pat. No. 3,647,917 and Nitzsche et al U.S. Pat. No. 3,886,118.
An improved, low odor, substantially non-corrosive one-package RTV composition is shown by Beers, U.S. Pat. No. 4,257,932, assigned to the same assignee as the present invention. Beers achieves a reduction in odor and corrosive properties by utilizing as a cross-linking silane, a less volatile material such as methyltris-(2-ethylhexanoxy)silane.
A non-corrosive two package moisture curable organopolysiloxane composition free of carboxylic acid generating groups is shown by Nitzsche et al, U.S. Pat. No. 3,127,363 which is based on the use of a polyalkoxysilane, or polysilicate cross linking agent, in place of methyltriacetoxysilane. The ingredients of the two package noncorrosive composition of Nitzsche et al, are mixed under atmospheric conditions and the resulting composition must be used soon after the ingredients are mixed because the resulting blend has a short shelf life. Although the mixture of Nitzsche et al, which is typically polyalkoxysilane, silanol-terminated polydiorganosiloxane and tin soap catalyst, provides upon mixing, a fast curing non-corrosive room temperature vulcanizable composition, the Nitzsche et al mixture does not have the extended shelf life advantage of the one package system which is required for various commercial uses and therefore is excluded from a variety of applications.
Nitzsche et al, U.S. Pat. No. 3,065,194, teaches that a mixture of an endblocked dimethylsiloxane polymer, such as hydroxy and alkoxy endblocked, inert filler, ethylorthosilicate and dibutyltindilaurate can be vulcanized upon contact with water, after a 14 day shelf period at room temperature. However, the various ingredients of the mixture have to be vigorously dried by heating for 1 hour at 200.degree. C., and the RTV, after a relatively short shelf period, has to be drenched with water.
Improved results toward combining the advantages of a non-corrosive acid-free polyalkoxysilane cross-linking agent with a silanol-terminated polydiorganosiloxane as a one-package system are shown by Weyenberg, U.S. Pat. No. 3,334,067, Cooper et al, U.S. Pat. No. 3,542,901 and by Smith et al U.S. Pat. Nos. 3,689,454, and 3,779,986, the last two being assigned to the same assignee as the present invention, utilizing a titanium chelate catalyst in place of a tin catalyst. However, after room temperature vulcanizable one-package systems based on a titanium chelate catalyst were allowed to age for a period of 5 hours or more, it was found that the tack-free time of the aged RTV was considerably longer than the tack-free time of the same mixture after it was initially mixed and immediately exposed to atmospheric moisture.
As shown by Brown et al U.S. Pat. No. 3,122,522, a platinum catalyst is used to prepare an alkoxy terminated silalkylenepolysiloxane polymer. However, this method of synthesizing the base polymer requires an expensive hydrosilylation procedure. Additional efforts to achieve a desirable non-corrosive, substantially odor-free stable one-package RTV based on the use of polyalkoxy organopolysiloxane in a more economic manner are shown by Brown et al, U.S. Pat. No. 3,161,614 or U.S. Pat. No. Re. 29,760. Brown et al employed a polyalkoxy end blocked polysiloxane which was based on the use of a mineral acid generating polyalkoxyhalosilane, and a curing catalyst. However, these compositions were found to be unusable because they failed to cure in contact with a tin catalyst, even in the presence of moisture.
As utilized hereinafter, the term "stable" as applied to the one package alkoxy-terminated organopolysiloxane RTV's of the present invention means a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.
In U.S. Pat. No. 4,395,426, filed June 26, 1981, White et al relating to alkoxy functional RTV compositions and process, room temperature vulcanizable compositions are shown based on the use of a polyalkoxy-terminated polydiorganosiloxane having the formula, ##STR2## where R and n are as previously defined, R.sup.1 is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkylether radicals, alkylester radicals, alkylketone radicals and alkylcyano or a C.sub.(7-13) aralkyl radical, R.sup.2 is a C.sub.(1-13) monovalent organic radical selected from R radicals as previously defined, and a is a whole number equal to 0 to 2 inclusive. The RTV composition of U.S. Pat. No. 4,395,526 further utilize a silicon-containing scavenger for materials having chemically combined hydroxy radical. The silicon containing scavengers can be silanes included within the formula, ##STR3## where R.sup.1 and R.sup.2 are as previously defined, X is a hydrolyzable leaving group selected from amido, amino, carbamato, enoxy, imidato, isocyanato, oximato, thioisocyanato and ureido radicals, b is a whole number equal to 0 or 1, and c is an integer equal to 1 to 4 inclusive and the sum of b+c is equal to 1 to 4 inclusive.
In Ser. No. 481,524, filed concurrently herewith, for Mary Ann White et al, One-Package, Stable, Moisture Curable, Alkoxy-Terminated Organopolysiloxane Compositions, there are shown room temperature vulcanizable organopolysiloxane compositions utilizing a silicon-free scavenger or organic scavenger for chemically combined hydroxy radicals in the RTV composition.
Although valuable results have been achieved by using such silicon containing scavengers or silicon-free organic scavengers for one-package RTV compositions, experience has shown that these one-package RTV compositions often require an extended period of time to achieve a tack-free state upon exposure to atmospheric moisture. Various amine accelerators have been found effective, such as di-n-hexylamine, silyl-substituted guanidines, or nitrogen bases, for example, amidine, aminoxides, aminopyridines, etc. However, none of the aforementioned organic nitrogen compounds were found to be sufficiently vigorous to effect a substantial increase in the cure rate of moisture curable alkoxy containing organopolysiloxane compositions, such as a factor of 4-6 times over that shown for the amine accelerators of the prior art.
The present invention is based on the discovery that certain organo nitrogen compounds, for example di-t-butylethylenediamine (DBEDA) and certain cyclic amidines having at least one chemically combined unit of the formula, ##STR4## where the free valences of such unit are satisfied by atoms selected from hydrogen, carbon, silicon and mixtures thereof are highly efficient curing accelerators. For example, there are included in formula (4) such cyclic amidines as 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). These particular organo nitrogen compounds have been found to provide a dramatic increase in RTV cure rates when used in combination with a condensation catalyst and either a silicon containing scavenger, or silicon-free organic scavenger, as defined hereinafter as compared to conventional amine accelerators such as di-n-hexylamine. In addition, it also has been found that the organo nitrogen curing accelerators of the present invention can be utilized in further combination with organic amines, such as di-n-hexylamine, to achieve further acceleration in cure rate.
There also can be utilized in the RTV compositions of the present invention cross-linking polyalkoxysilane having the formula, ##STR5## where R.sup.1, R.sup.2 and b are as previously defined.
As a result of the employment of the organo nitrogen accelerators of the present invention in one-package moisture curable alkoxy-terminated organopolysiloxane compositions, RTV compositions are provided which are stable over an extended period of time and which are convertible upon exposure to atmospheric moisture to the tack-free state in a substantially reduced period of time.